For coding a picture of a video sequence, video compression methods usually divide the picture into a set of blocks of pixels. Each block is then predicted using information already reconstructed, corresponding to the blocks previously encoded/decoded in the current picture. The coding of a current block is performed using an intra or inter prediction of the current block, and a prediction residual or “residual block” corresponding to a difference between the current block and the predicted block is computed. The resulting residual block is then converted, for example by using a transform such as a DCT (discrete cosine transform) type transform. The coefficients of the transformed residual block are then quantized and encoded by entropy coding and transmitted to a decoder.
In an HEVC video compression standard (“ITU-T H.265 Telecommunication standardization sector of ITU (10/2014), series H: audiovisual and multimedia systems, infrastructure of audiovisual services-coding of moving video, High efficiency video coding, Recommendation ITU-T H.265”), a picture is divided into Coding Tree Units (CTU), which size may be 64×64, 128×128 or 256×256 pixels. Each CTU may be further subdivided using a quad-tree division, where each leaf of the quad-tree is called a Coding Unit (CU). Each CU is then given some Intra or Inter prediction parameters. To do so, a CU is spatially partitioned into one or more Prediction Units (PU), a PU may have a square or a rectangular shape. Each PU is assigned some prediction information, such as for example motion information, spatial intra prediction. According to the HEVC video compression standard, each CU may be further subdivided into Transform Units (TU) for performing the transform of the prediction residual. However, only square supports transform are defined in the HEVC video compression standard, as disclosed on FIG. 1A. On FIG. 1A, solid lines indicate CU boundaries and dotted lines indicate TU boundaries.
A Quad-Tree plus Binary-Tree (QTBT) coding tool (“Algorithm Description of Joint Exploration Test Model 3”, Document JVET-C1001_v3, Joint Video Exploration Team of ISO/IEC JTC1/SC29/WG11, 3rd meeting, 26 May-1 Jun. 2015, Geneva, CH) provides a more flexible CTU representation than the CU/PU/TU arrangement of the HEVC standard. The Quad-Tree plus Binary-Tree (QTBT) coding tool consists in a coding tree where coding units can be split both in a quad-tree and in a binary-tree fashion. Such coding tree representation of a Coding Tree Unit is illustrated on FIG. 1B, where solid lines indicate quad-tree partitioning and dotted lines indicate binary partitioning of a CU.
The splitting of a coding unit is decided on the encoder side through a rate distortion optimization procedure, which consists in determining the QTBT representation of the CTU with minimal rate distortion cost. In the QTBT representation, a CU has either a square or a rectangular shape. The size of coding unit is always a power of 2, and typically goes from 4 to 128. The QTBT decomposition of a CTU is made of two stages: first the CTU is split in a quad-tree fashion, then each quad-tree leaf can be further divided in a binary fashion or in a quad-tree fashion, as illustrated on FIG. 1C, where solid lines represent the quad-tree decomposition phase and dotted lines represent the binary decomposition that is spatially embedded in the quad-tree leaves. With the QTBT representation, a CU is not anymore partitioned into PU or TU. With the QTBT representation, the transform of the prediction residual is performed on blocks of size expressed as a power of 2 and existing separable transform and fast implementation of such transform usually used for square blocks can be re-used. However, such a QTBT representation does not allow for asymmetric splitting of a CU.